Solving Autogenous Shrinkage for Greener Concrete

There are a few definitions of autogenous shrinkage but essentially is associated with a deficit of water for the hydration of cement. Because of the polar nature of water a nano - layer is adsorbed on to many surfaces including minerals which if drawn away by chemical reaction will cause shrinkage. Neville describes this rather too simply as a "withdrawal of water from the capillary pores"[1] referring to a concrete matrix that has become a closed system.

Any compound with a surface charge (positive, negative or both of appropriate spacing) will attract water that will stick to it as a result of what are called Van der Waals or polar bonds which are of an electrostatic nature. This adsorbed water layering, often referred to as hydration shelling or Van der Waals shelling can be one to many layers deep and has been shown by Wang[2] to be very deep around brucite the mineral that forms as a result of the hydration of magnesia. Reduce the amount of this adsorbed water by consuming water as a result of chemical reaction and shrinkage occurs as the nano layers disappear on a compounding basis..

The Gibbs free energy describes how readily reactions proceed on a molar basis however does not describe the demand/transport of water molecules or whether this propensity to react is greater than the propensity to merely stick to charged surfaces as a result of hydrogen bonding reduced by distance. We have asked several eminent geochemists and at this point in time nobody has developed suitable models or math. For bright young people it would be an interesting and challenging project because then we could predict the amount of autogenous shrinkage and mitigation using for example brucite hydrates to supply an alternative source of water.

The setting and hardening of concrete is far from a rapid process and it takes years for ultimate strength to develop. Conventionally we add water only at the beginning or during the early life of a concrete and are limited in how much can be added if strength is not to be compromised. As concretes dry out and hydration proceeds a shortage of water available at reaction sites for more complete hydration of PC becomes manifest.

Few people are aware that around 18-20% of the Portland cement in most concretes does not hydrate anyway as the concrete dries out first and there is insufficient water left. It follows that virtually all concretes have chemical shrinkage as a result of latent water demand for more complete hydration and for practical purposes this is often not separated from drying shrinkage.

Hydration is a slow process and the key to solving chemical or autogenous shrinkage is to have water available for the duration and brucite hydrates perform this role.

A Structural Model of Brucite Hydrate (Yellow = Mg, White=Hydrogen, Red = Oxygen

One of the reasons TecEco advocate the use of a small percentage of reactive magnesia with all dense concretes is that when it hydrates if forms brucite hydrates not brucite as most assume. Brucite hydrates have additional polar bound water between layers of magnesium hydroxide which is available for the more complete hydration of PC. More complete hydration of PC means that less can be used reducing costs and saving the environment. With reduced dimensional distress as a result of less autogenous shrinkage concretes are much more durable and for this reason also more sustainable [3].

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[1] Neville, A. M. (1995). Properties of Concrete. England, Pearson Education Limited at page 425.

[2] Wang, J., A. G. Kalinichev, et al. (2004). "Molecular modeling of water structure in nano-pores between brucite (001) surfaces." Geochimica et Cosmochimica Acta 68(16): 3351.

[3] Hawken, P. (1993). The Ecology of Commerce. New York, Harper Collins.